The invention relates to an electroluminescent device comprising an active layer of a semiconducting conjugated polymer, which layer is present between a first and a second pattern of electrodes, at least one of the two patterns being transparent to the light to be emitted and a first pattern comprising a material which is suitable for injecting holes in the active layer.
The active layer and the two electrode layers may jointly constitute one light-emitting diode (LED), but the electroluminescent device preferably comprises a plurality of LEDs, for example in the form of a matrix of light-emitting surfaces as intended for a display.
Solid-state LEDs may be used as illumination sources and in, for example displays and indicator lamps. For the active layer in such structures, traditional III-V semiconductor compounds are generally used, such as doped GaAs and GaP. The operation is based on the recombinations of electron hole pairs which are injected in the semiconductor material from electrodes located on both sides of the active layer. Due to these recombinations, energy in the form of (visible) light is released, which is a phenomenon referred to as electroluminescence. The wavelength and hence the colour of the emitted light is determined by the bandgap of the semiconductor material.
The use of semiconducting organic polymers such as proposed in an article by D. Braun and A. J. Heeger in Applied Physics Letters 58 (18), pp. 1982-1984 (May 6, 1991) increases the number of possible materials for use in these types of devices. Semiconducting organic polymers have a conjugated polymer chain. The bandgap, the electron affinity and the ionization potential can be adjusted by suitable choice of the conjugated polymer chain and by the choice of suitable side chains. In contrast to electrically conducting polymers, these conjugated polymers are undoped. An active layer of such a polymer material may be manufactured by means of a CVD process, but is preferably manufactured by spin-coating of a solution of a soluble conjugated polymer. With these processes, LEDs and displays having a large light-emitting surface can be manufactured in a simple manner.
Matrix displays for displaying information, for example for video applications and monitors are divided into a large number of pixels which are arranged in rows and columns. Problems usually occur, notably when driving this type of matrix display. A first problem which may present itself is known as crosstalk at which a variation of the voltage at a row or column electrode has the effect of an unintentional voltage change across a (non-selected) pixel. Another problem which occurs particularly in larger matrixes such as used for, for example video information or datagraphic information, is the problem of flicker. Moreover, it is desirable, notably in video applications, that different values of light intensity (grey levels) can be emitted by the pixels.